Several lines of evidence suggest that the E2F family of transcription factors is important in cell growth control: 1) the E2F family regulates many genes required for DNA synthesis and cell cycle progression; 2) mutations in the E2F signal transduction pathway are found in many cancers; 3) over-expression of E2F family members can alleviate growth factor requirements and lead to tumorigenicity, suggesting that E2F family members are positive regulators of cell growth; and 4) the increased number of tumors that are observed in a E2F1 nullizygous mouse suggests that E2F1 may also be a tumor suppressor. These two seemingly conflicting functions of E2F are likely due to the ability of E2F family members to be both activators and repressors of transcription. Models for cell cycle regulation in which E2F mediates a positive role in cell growth focus on the S phase-specific activation of E2F target genes, whereas models that invoke a tumor suppressor function for E2F focus on the GO phase-specific repression of E2F target genes. We propose to use the mouse liver, a well- characterized in vivo model for studying the regulation of cell proliferation and tumorigenicity, to determine if the main role of E2F is to function as an activator or a repressor or chemically-induced liver neoplasia. We will utilize E2F1 nullizygous mice (Aim I) and transgenic mice that express a dominant negative E2F1 (Aim III) to analyze the effects of reducing E2F activity. E2F target gene expression in these mice will differ depending on whether the promoter is most influenced by GO phase repression or S phase activation. The goals of these experiments are to determine which category of E2F target genes is most critical for mediating the role of E2F in the hepatocyte. We will use a transgenic mouse that expressed and E2F1 derivative that can derepress GO phase- specific transcription but cannot activate S phase-specific transcription of E2F target genes (Aim II). The goals of this Aim are to determine it derepression of E2F target genes is sufficient to cause neoplastic transformation of hepatocytes. We also propose experiments (Aim IV) in which we will identify and characterize examples of the two classes of E2F target promoters; this is essential for understanding the phenotype of the E2F derivatives that we are using in our animal model systems. In summary, the long-term goals of our experiments are to determine the mechanism by which the E2F family mediates cell growth control in both normal liver and chemically-induced hepatocarcinogenesis.